This invention relates to a data transfer method for use in a semiconductor data recording medium, specifically a data transfer method suitable to transfer data between an IC card and a reader/writer device without physical contact therebetween.
Recently IC cards have been noted as a new data recording medium. IC cards are a card-shaped recording medium which is convenient to be carried with, while they contain semiconductor recording devices built in and can store very large volumes of data in comparison with magnetic cards. The IC cards having a built-in CPU are very useful in applications requiring high security because of the arithmetic processing function they own.
Reader/writer devices are used to write and read data in and from semiconductor data recording mediums such as the above-described IC cards, and so on. The input/output terminals of the reader/writer devices are electrically contacted to the input/output terminals of the IC cards, whereby electric power and a clock can be supplied from the reader/writer devices to the IC cards, and data transfer is possible between them. Recently a new method for transferring data between them without any physical contact with each other has been proposed. That is, a reader/writer device and an IC card are magnetically coupled to each other by means of a first coil of the former and a second coil of the latter, whereby an electric power and a clock are supplied, and data transfer are conducted. Such non contact data transfer dispenses with external input/output terminals, which makes IC cards more portable.
But the data transfer method used in the above-described conventional semiconductor data recording medium has a problem that accurate data transfer cannot be conducted. Generally noncontact data transfer is very disadvantageous in conducting accurate data transfer in comparison with the data transfer in secured electric contact. That is, in order to conduct noncontact data transfer, a carrier is modulated in accordance with a data signal, then the modulated wave is transmitted from a first coil to a second coil by the electromagnetic induction, and then the modulated wave is demodulated to extract the data signal. Resultantly the demodulated data signal has a considerably rounded waveform, and it is more possible that the so-called bit falls occur. In addition, external noises tend to mix in. Accordingly to make highly reliable data transfer it is necessary to conduct sufficient error detecting process and others. Furthermore, to make two-way data transmission, in the conventional method two coils, a transmission coil and a receiving coil, have to be provided on each of the reader/writer device and the semiconductor data recording medium. But the semiconductor data recording medium, such as IC cards, must make use of a characteristic of being a small-sized device which is conveniently portable. It is difficult for the IC card to include a couple of coils for efficient magnetic coupling.
Another problem of the conventional noncontact data transfer is that the data transfer from the reader/writer device to the semiconductor data recording medium, and that from the latter to the former cannot be concurrently made. Consequently it is necessary that a period of transfer time is time-divided to make a transfer alternately to one way and to the other way in such a manner that when a transfer to one way is over, that to the other way is conducted. Consequently no quick response cannot be made between both in terms of data transfer.
In the system in which noncontact data transfer is conducted, particular reset means is needed. That is, when the IC card is reset on the side of the reader/writer device, some external reset command has to be transferred to the IC card. In a contact data transfer system a special reset line is provided between the two, and an external reset command signal can be transmitted through the reset line. But in the noncontact data transfer system, data transmission channel has to be formed between the two by the magnetic coupling between the two coils. Consequently to form a special reset channel between the two, an additional coil has to be provided to form an additional magnetic coupling. But unpreferably the provision of an additional coil hinders the miniaturization of the IC card.
In the conventional noncontact data transfer system, reset is conducted by temporarily discontinuing a power supply to the IC card. That is, the IC card is reset by re-actuating the data transfer system. But a problem of this method of resuming an electric supply following a pause thereof is that the system takes time to have its stable state.